An interesting case of a giant renal hydatid cyst is presented. The big cystic mass detected at ultrasonography (US) and computerized tomography (CT) in a ten-year-old girl looked like a simple cyst. No germinative membrane or any other radiological sign of a hydatid cyst was present. The operation was planned for a simple cyst; but was altered, after an intraoperative diagnostic needle sampling brought out the characteristic fluid of hydatid cyst. We came to the conclusion that hydatid cystic masses may not always present with their characteristic radiological findings; and that extreme caution should be practiced by the radiologist and the surgeon in order to prevent iatrogenic echinococcal dissemination.